The one of the five general-purpose resins, polyvinylchloride (PVC) is used for various applications, such as plastics, films, adhesives, etc. and, primarily for plastic sols. The most popular plastic sol for the industrial use purpose is the PVC sol that is prepared by dispersion of PVC powder and fillers in a plasticizer. The PVC sol contains, according to its use purpose, a pigment, a thermal stabilizer, a foaming agent, a diluent, and so forth.
The plastic sol is widely used in various applications, including automobile, carpet, wallpaper, coating, or the like. In particular, the PVC sol is generally applied to automobile underbody floor, wheel housing, or fuel tank for the purpose of water tightness, vibration isolation, and anticorrosion, and automobile body panel junction and hood, door, or the like for the purpose of water tightness and anticorrosion. The PVC sol, however, gives off a hydrogen chloride gas from its principal component, PVC during incineration to cause damage on the incinerator and generation of dioxin as well as acid rain and destruction of the ozone layer. Due to these problems, the restriction on the use of the PVC sol has been consolidated with many attempts made on the substitute of the PVC sol.
Accordingly, Japanese Patent Laid-open Nos. H07-102147, 08-3411 and 08-73601 disclose a method for preparing an acrylic polymer and an acrylic sol containing the acrylic polymer. According to the method, the acrylic polymers for sol have a double-layer or multi-layer core/shell structure that the core layer is comprised of a polymer highly compatible with a plasticizer, the shell layer being comprised of a polymer poor in compatibility with the plasticizer. The acrylic polymer particle of the core/shell structure gets storage stability from the shell layer retarding gelation caused by the plasticizer at the storage temperature, and forms a coating from the polymer of the core layer highly compatible with the plasticizer at the gelation temperature.
But the acrylic polymer beads as disclosed in those documents have a wide particle size distribution in the range of 0.1 to 100 μm and are not suitable to an acrylic sol for automobile requiring high storage stability.
More specifically, when the acrylic polymer obtained after spry drying contains beads having a particle diameter of less than 10 μm, the particles are readily plasticized at the room temperature after the preparation of the acrylic sol to rapidly increase the viscosity of the sol with an elapse of time. The reason why these spray-dried beads have a wide particle size distribution is because the primary particles in the polymerized emulsion state and hence the secondary particles formed after the spray drying have a wide particle size distribution. Moreover, the high water absorption capacity characteristic to the acrylic polymer causes bubbles in the coating formed by gelation and leads to serious problems when coated on the mobile underbody floor, or wheel housing.
To summarize the problems with the prior art, the acrylic polymer beads according to the conventional method for preparing acrylic polymer beads having a core/shell structure have a wide particle diameter distribution and contain particles having a particle diameter of less than 10 μm, thereby hardly providing storage stability of the sol. In addition, the high water absorption capacity of the acrylic polymer particles causes bubbles in the coating formed by gelation of the acrylic sol to deteriorate the properties of the coating.
In an attempt to solve the problems related to the poor storage stability of the acrylic sol and the deteriorated properties of the coating formed by gelation in the preparation of acrylic polymer beads having a core/shell structure, the inventors of the present invention have figured out that a core/shell type acrylic polymer emulsion having a narrow particle diameter distribution of 0.2 to 0.5 μm can be prepared by using a part of the monomer constituting a core layer to form seeds by a seed polymerization, polymerizing the rest of the monomer to form a core layer and then forming a shell layer containing an appropriate amount of a crosslink agent to form acrylic polymer beads having a core/shell structure.
The inventors of the present invention have also figured out that acrylic polymer beads having a particle diameter of 10 to 100 μm can be prepared by spray-drying the core/shell type acrylic polymer emulsion and used for the preparation of an acrylic sol having a high storage stability and excellent properties of the coating formed by gelation.
It is therefore an object of the present invention to provide acrylic polymer beads having a narrow particle size distribution.
It is another object of the present invention to provide a method for preparing acrylic polymer beads having a core/shell structure and a narrow particle size distribution by seed polymerization.
To achieve the objects of the present invention, there is provided acrylic polymer beads for automobile having an average particle diameter of 0.2 to 0.5 μm, a standard deviation of particle diameter of 1 to 12% with respect to the average particle diameter as measured with a Submicron Particle Sizer in an emulsion obtained by emulsion polymerization, and the final average particle diameter of 10 to 100 μm.
In one aspect of the present invention, there is provided a method for preparing acrylic polymer beads for automobile that uses an emulsion polymerization and involves preparing an emulsion containing acrylic polymer beads having a core/shell structure, and spray-drying the emulsion to form acrylic polymer beads. The method includes: (a) adding an ion-exchange water, 5 to 60 wt % of a monomer constituting a core layer, and an emulsifier, heating the resultant mixture, adding an aqueous initiator, and performing polymerization for 1 to 4 hours to form seeds; (b) adding dropwise the rest of the monomer constituting the core layer, and further performing polymerization for 1 to 6 hours to form a core layer; (c) adding a monomer constituting a shell layer, and further performing the polymerization for 2 to 8 hours to form an emulsion containing acrylic polymer beads having a core/shell structure; and (d) spray-drying the emulsion containing the acrylic polymer beads to prepare the acrylic polymer beads.
In another aspect of the present invention, there is provided an acrylic sol composition for automobile that includes 100 parts by weight of the acrylic polymer beads having a core/shell structure as obtained by the above-described method; 50 to 150 parts by weight of a plasticizer; and 50 to 150 parts by weight of a filler.